Lubrication film topography

PhD of Axel Huerre

Collaboration with O. Theodoly and M..P. Valignat (LAI), I. Cantat (IPR) and A. Leshansky (Technion Univ.)

We study the motion of droplets in a confined, micrometric geometry, by focusing on the lubrication film between droplet and wall. When capillary forces dominate, the lubrication film thickness evolves non linearly with the capillary number due to viscous dissipation between meniscus and wall.  However, this film may become thin enough (tens of nanometers) that intermolecular forces come into play and affect classical scalings. We report the novel experimental characterization of two dynamical regimes as the capillary number increases: (i) at low capillary numbers, the film thickness is constant and set by the disjoinging pressure, while (ii) above a critical capillary number, the interface behavior is well described by a Bretherton-like viscous scenario. At a high surfactant concentration, structural effects lead to the formation of patterns on the interface, which can be used to trace the interface velocity. Our experiments yield highly resolved topographies of the shape of the interface and allow us to bring new insights into droplet dynamics in microfluidics. 

Huerre et al., Phys. Rev. Lett., 2015.
Huerre et al., Lab. Chip, 2016.

Laplace pressure based disjoining pressure isotherm in non

symmetric conditions

PhD of Axel Huerre

Collaboration with O. Theodoly and M..P. Valignat (LAI), T. Maggs (Gulliver)

Understanding the stability and dynamics of two phase systems, such as foams and emulsions, in porous media is still a challenge for physicists and calls for a better understanding of the intermolecular interactions between interfaces. In a classical approach, these interactions are investigated in the framework of DLVO theory by building disjoining pressure isotherms. The paper reports on a technique allowing the measurement of disjoining pressure isotherms in  a thin liquid film squeezed either by a gas or a liquid phase on a solid substrate. We couple a Reflection Interference Contrast Microscopy (RICM) set-up to a microfluidic channel that sets the disjoining pressure through the Laplace pressure. This  simple technique is found to be both accurate and precise. The Laplace pressure mechanism provides extremely stable conditions and offers opportunity for parallelizing experiments by producing several drops in channels of different heights. We illustrate its  potential by comparing experimental isotherms for oil - (water and SDS) - glass systems with different models focusing on the  electrostatic contribution of the disjoining pressure. The extracted values of the interface potentials are in agreement with the constant surface potential model and with a full computation. The derived SDS surface concentration agrees with values reported in the literature. We  believe that this technique is suitable to investigate other working fluids and intermolecular interactions at smaller scales.

Huerre et al., Applied Physics Letters, 2017.

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